Ferguson, Missouri (CNN) — [Breaking news, posted at 9:34 p.m. ET]

A Missouri grand jury has decided not to indict Ferguson police Officer Darren Wilson in the August 9 shooting death of Michael Brown, St. Louis County Prosecuting Attorney Robert McCulloch said Monday night.

After an “exhaustive review,” the jurors deliberated for two days, he said. The grand jurors are “the only ones who have heard all the evidence,” McCulloch said.

Brown’s father is “devastated” that Wilson will not face charges, a spokeswoman for Michael Brown Sr. told CNN’s Evan Perez.

“While we understand that many others share our pain, we ask that you channel your frustration in ways that will make a positive change. We need to work together to fix the system that allowed this to happen,” the family said in a statement.

[Original story, posted at 9 p.m. ET]

While protesters chanted late Monday for an indictment in the shooting death of Michael Brown, officials called for calm as the world awaits word of the grand jury’s decision.

“No matter what is announced, people will be emotional. I want people to think with their heads and not with emotion,” said St. Louis County Executive Charlie Dooley. “This is not the time to turn on each other. It is a time to turn to each other.”

Protesters were gathering outside the police department in Ferguson. Darren Wilson, a white police officer with the department, shot and killed Brown, a black teenager, on August 9.

The grand jury’s decision on whether to indict Wilson will be announced at about 8 p.m. (9 p.m. ET) Monday, according to the Office of the Prosecuting Attorney of St. Louis County.

“While none of us knows what that will be, our shared hope and expectation is that regardless of the decision, people on all sides show tolerance, mutual respect and restraint,” Gov. Jay Nixon told reporters.

As he spoke, protesters outside the police department chanted: “Indict that cop!” and “No justice, no peace!” A line of police guarded the entrance.

Members of the National Guard will be in the area to provide security at firehouses, police stations and utility substations, the governor said.

Brown’s death ignited a national debate on race and law enforcement. Nowhere was the tension more evident than in the predominantly black town of Ferguson, which has a mostly white police department and town government.

A source close to Wilson told CNN’s Don Lemon that the officer’s representatives — through a process of deduction — have determined that there will be no indictment.

At this point, if there was going to be an indictment, Wilson would have been asked to surrender, the source said late Monday.

Supporters of Brown’s family back witness accounts that Wilson fired while Brown, 18, had his hands up in surrender. Wilson’s supporters say that Brown was the aggressor and had tried to take Wilson’s gun while he was in his vehicle and that the officer fired in self-defense.

The town couldn’t even agree what happened during weeks of street demonstrations. Protesters argued that authorities were trying to stifle protests; officials said they were acting to keep violence under control.

Brown’s family is asking for four and a half minutes of silence before any protests begin, family representative Janie Jones said.

“We are not here to be violent. We are here in memory of our son. We are here for protection of all children. We are here to support justice and equality for all people. We lift our voices to ensure black and brown men, women and children can live in this country without being devalued because of the color of our skin,” the family said in a statement.

The time period is a reference to the approximately four and a half hours Brown’s body remained in the street after he was shot.

An investigation by the St. Louis Post-Dispatch found that county detectives were 30 miles away when they were called, and the first detective arrived about 90 minutes after the shooting. When a hearse later arrived, an angry crowd had gathered and it was too dangerous to get the body, authorities said. Ultimately, a SWAT team came.

“The world will be watching us. They’re going to watch how we handle our disagreements in the coming days and how we make needed change in the coming months and years,” St. Louis Mayor Francis Slay told reporters.

‘We’re just ready for it to be over with’

The city hasbeen on edgein anticipation of a decision. Law enforcement sources said the grand jury was sent home.

“I’m already angry because it has taken so long,” he said. “If Mike Brown had shot Darren Wilson, it would have been over a long time ago. But Darren Wilson is walking around free. He hasn’t apologized to Mike Brown’s family. He has not apologized to the community.”

Though the basic facts of the case — that Brown was unarmed when Wilson shot him — are not in question, the facts of the fatal moment are hotly disputed. Authorities have said Wilson stopped Brown because Brown and a friend were blocking traffic, by walking down the middle of the street.

Protests in Ferguson might be hampered by winter weather, with temperatures expected to drop into the upper 20s Monday night.

The Ferguson-Florissant School District has canceled classes and activities for Tuesday as a result of the expected grand jury announcement. The district’s calendar shows the schools will be closed from Wednesday through Friday for Thanksgiving. The nearby Riverview Gardens School District similarly canceled school for Tuesday.

Unrest could flare in other cities, too, in the wake of the grand jury decision, Rashid Abdul-Salaam, a security specialist and former police officer, told CNN.

“Departments would be remiss if they didn’t prepare,” he said.

Unanimous indictment decision isn’t needed

Unlike a jury in a criminal case, which convicts someone if jurors are convinced of guilt “beyond a reasonable doubt,” a grand jury decides whether there is “probable cause” to charge someone with a crime, based on testimony and evidence presented.

In Missouri, grand jurors don’t have to be unanimous to indict, as long as nine of the 12 agree on a charge.

The grand jury can issue an indictment on any of these four charges: First-degree murder, second-degree murder, voluntary manslaughter and involuntary manslaughter. It also can add a charge of armed criminal action, authorities said.

But the grand jury, which received the Missouri statutes for self-defense and the police use of deadly force, may choose not to indict Wilson.

The grand jury meets in secrecy and first met in August. While the jury members are not identified, authorities have released some information about them.

The group of 12 includes nine white people (six men and three women) and three black people (two women and one man), court officials said.

The county grand jury was randomly selected from an approved pool and has been seated since May, according to Paul Fox, director of judicial administration for the St. Louis County Circuit Court.

Many grand juries hear numerous cases. These juries last for a specified period of time rather than the duration of a specific case.

Wilson himself testified before the grand jury, CNN legal analyst Jeffrey Toobin said, an unusual move because he gave up his Fifth Amendment rights in doing so.